1. Field of the Invention
The invention pertains to a method and a device for printing on one surface of a strip-shaped medium, the other surface of which carries a self-adhesive coating, the strip being transported in its longitudinal direction through a printing mechanism, in which the surface of the medium to printed is guided past a print head while the surface with the self-adhesive coating is guided over a driven roller, and to a device suitable for implementing the method.
2. Description of the Related Art
Methods and devices of the type in question are used especially for producing the printed labels which are affixed to items of merchandise in retail sales establishments. In particular, weight information such as the that originating from a self-service scale, price information, and/or barcodes, i.e., information which is important for the handling of sales transactions, is printed on the surface to be printed. The circumstance that the surface with the self-adhesive coating is freely exposed as it is being guided through the printing operation offers an advantage over the conventional method, in which the surface with the self-adhesive coating is provided with a liner strip, from which the printed labels must be peeled off before they can be applied, the advantage being that the medium supplied to the printing operation, usually in the form of a roll, takes up less space, and at the same time, the costs associated with the liner strip are also eliminated. The handling of the medium with the exposed self-adhesive surface, however, requires special measures during the printing operation to prevent the medium from getting permanently stuck during the printing operation.
In known methods and devices of the type indicated above (EP 0 758 979 B1 and EP 0 758 955 B1), this problem is dealt with in that the roller and also the other surfaces which come in contact with the self-adhesive coating during the printing operation are provided with an adhesive-repellent coating. This is not sufficient, however, to ensure that the medium can be separated reliably from the roller. For this reason, a release blade or a small-diameter release roller is installed as an especially important element downstream in the transport direction from the roller and immediately adjacent to it, the surface of the release blade or roller being provided with an adhesive-repellent coating. The strip-shaped medium is thus directed away from the roller as it passes over this release element.
The invention is based on the task of providing a method and a device of the type indicated above which are characterized by unusual simplicity and reliability.
With respect to the method, this task is accomplished by impressing into the medium a shape behavior which allows the medium to separate itself from the roller.
The strip-shaped medium, which is usually a paper suitable for thermal printing, has the tendency to follow the roller with which its self-adhesive surface is in contact for the purpose of being transported; that is, the medium tends to wrap itself around the roller unless measures are taken downstream from the roller in the transport direction to separate the medium from the roller. In the solution according to the invention, no measures of this type are needed. Instead, the shape behavior impressed into the medium ensures that it will separate itself from the roller as soon as it is no longer being guided to keep it in contact with the roller. In most cases it is the print head which provides this guidance; the print head is usually located at a tangent to and opposite the roller and exerts pressure on the surface of the medium to be printed, thus pressing it toward the roller. After the medium has been transported between the print head and the roller, this guidance is no longer present, as a result of which the medium can unfold the shape behavior impressed into it, which allows it to assume a shape which deflects it away from the roller.
A first basic embodiment consists in that a curvature in the longitudinal direction is impressed into the medium during transport upstream of the roller, the center of this curvature being opposite the surface to be printed.
This embodiment takes advantage of the intrinsic stiffness of the strip-shaped medium, which causes the medium to try to retain the curvature impressed into it. Even if, after the curvature has been impressed, the medium is stretched out straight again as it is being transported to the roller, it assumes this curvature again after it has passed beyond the roller. This means that it curves away from the roller and thus separates from it reliably.
An effective elaboration of this basic embodiment consists in that the surface of the medium to be printed is transported over an impressing cylinder with a radius suitable for the purpose around a wrap angle suitable for the impressing. While the medium is passing around the impressing cylinder, the desired curvature is impressed continuously. The desired degree of curvature can be adjusted by selecting the wrap angle and the radius of the impressing cylinder.
A second basic embodiment of the invention consists in that a rib extending in the longitudinal direction is impressed into the medium before it leaves the driven roller.
This rib increases the longitudinal stiffness of the strip-shaped medium and increases its tendency to stretch itself out straight. As the strip-shaped medium passes over the roller, it therefore separates approximately at a tangent from the roller, which means that the medium is prevented from wrapping itself around the roller.
It is advisable for this basic embodiment to be implemented in such a way that a rib is impressed into the medium along each of the two edges parallel to the longitudinal direction. The ribs impressed into the edges of the medium provide a significant increase in the tendency of the medium to straighten itself out, but they also leave the entire space between the edges free for printing. The profiling of the medium caused by the ribs therefore does not interfere with the printing in the free area.
In this version of the method according to the invention, the situation can be allowed in which a curvature has already been impressed into the medium in its longitudinal direction before it passes through the printing mechanism, the center of this curvature being opposite the surface with the self-adhesive coating. This curvature is usually the result of supplying the strip-shaped medium from a roll, in which it is wound up in such a way that the surface with the self-adhesive coating faces the center of the roll. As it reaches the roller, the strip-shaped medium is therefore already curved in such a way that it tends to wrap itself around the roller. As a result of the ribs impressed into the medium, however, the strip straightens out precisely in opposition to this curvature and therefore separates itself reliably from the roller.
The basic principle of a device according to the invention suitable for implementing the method according to the invention is stated in claim 7. Embodiments of the invention suitable for implementing the variants of the method discussed under the first basic embodiment are indicated in claims 8-15, and devices suitable for implementing the variants of the method discussed under the second basic embodiment are specified in claims 16-19. The device features of claims 20-22 can be applied effectively in all cases.